This invention relates to an ignition timing control apparatus for an internal combustion engine. More particularly, it relates to an ignition timing control apparatus which can prevent abnormal ignition from occurring when the engine rotational speed suddenly increases.
An internal combustion engine is generally equipped with a rotational position sensor which indicates the position of the piston of each cylinder of the engine. FIG. 1 illustrates the output signal of a typical rotational position sensor. The output signal is in the form of pulses which are generated at prescribed intervals, such as once every 180 degrees of crankshaft rotation. Each pulse corresponds to a specific cylinder of the engine and indicates a prescribed position with respect to top dead center of the piston of the cylinder. For example, in FIG. 1, the rising edge of each pulse occurs when the piston of the corresponding cylinder is at 75.degree. BTDC, while the falling edge occurs when the piston is at 5.degree. BTDC. The ignition timing for each cylinder is normally measured from the rising edge of the corresponding pulse, and ignition is controlled so as to occur a prescribed length of time after the occurrence of the rising edge of a pulse.
FIG. 2 illustrates the output signal of a conventional rotational position sensor for an engine and the current in the ignition coil of the engine. In this example, the ignition timing is set so that ignition occurs prior to the falling edge of an output pulse of the rotational position sensor, i.e., prior to 5.degree. BTDC. However, when the engine rotational speed suddenly increases, if the ignition timing was calculated before the speed increase took place, ignition will end up taking place later in the engine cycle than intended, and may even take place after the falling edge of a pulse, as shown by the dashed lines in FIG. 2. In this case, proper combustion will not take place, and the engine will not generate maximum power.
In order to prevent late ignition when the engine speed suddenly increases, some engines are equipped with a so-called "ignition timing braking device". If ignition has yet to take place at the time of the falling edge of the output pulse of the rotational position sensor, the ignition timing braking device cuts off the ignition coil current and forces ignition to occur. The process of cutting off the ignition coil current in this manner is referred to as "ignition timing braking". In the example of FIG. 2, when ignition timing braking is performed, ignition will take place no later than 5.degree. BTDC.
In the example of FIG. 1, the output pulses of the rotational position sensor are all identical. However, in some automobiles, the rotational position sensor generates a different type of output signal which enables identification of a prescribed reference cylinder of the engine. As shown in FIG. 3, which illustrates such an output signal, the number of degrees of crankshaft rotation between the rising and falling edges of a pulse is different for the reference cylinder than for the other cylinders. For example, in FIG. 3, the falling edge of a pulse corresponding to the reference cylinder occurs at 15.degree. BTDC, whereas it occurs at 5.degree. BTDC for the other cylinders. Thus, the falling edge of the pulse corresponding to the reference cylinder is offset by 10.degree. with respect to the other pulses. The duty cycle of a pulse corresponding to the reference cylinder is therefore different from the duty cycle of pulses for the other cylinders. By monitoring the duty cycle, the reference cylinder can be identified.
However, it is difficult to perform ignition timing braking with an ignition system which generates a rotational position signal of the type illustrated in FIG. 3. Therefore, with this type of rotational position signal, misfiring and other ignition problems occur when the engine rotational speed suddenly increases.